1. Field of the Invention
The present invention relates to a driving device in which a driven member is frictionally engaged with a drive shaft and moved according to displacements of the drive shaft caused by expansion and contraction of a piezoelectric element, an image capture apparatus and a mobile phone having the driving device.
2. Description Related to the Prior Art
A piezoelectric actuator, which utilizes displacements of a piezoelectric element by applying a voltage, is known. There is a piezoelectric actuator which moves a driven member in a predetermined reference-axis direction by allowing the piezoelectric element to displace at high speed and low speed alternately. Such piezoelectric actuator is applied to driving devices of precision instruments and the like, for instance, a lens driving device of a camera (see U.S. Pat. Nos. 5,225,941 and 5,589,723).
The piezoelectric actuator disclosed in the above references includes a drive shaft and the piezoelectric element fixed to one end of the drive shaft. A lens holding member, which holds a lens, is frictionally engaged with the drive shaft. When the piezoelectric element is displaced at the high speed, a dynamic friction between the drive shaft and the lens holding member is low so that the lens holding member slips along the drive shaft, and stays at the same position by inertia. When the piezoelectric element is displaced at the low speed, a static friction between the drive shaft and the lens holding member is high so that the lens holding member moves in the axial direction according to the displacement of the drive shaft.
The lens holding member is moved toward the piezoelectric element when the piezoelectric element is expanded at the high speed and contracted at the low speed. On the contrary, the lens holding member moves away from the piezoelectric element when the piezoelectric element is contracted at the high speed and expanded at the low speed. A drive signal applied to the piezoelectric element is a triangular pulse of a saw-tooth shape which has a period in which the positive or negative voltage rises quickly and a period in which negative or positive voltage rises slowly. For instance, when the positive voltage is applied to the piezoelectric element during the period, in which the voltage rises quickly, the piezoelectric element is expanded at the high speed. When the negative voltage is applied to the piezoelectric element during the period, in which the voltage rises quickly, the piezoelectric element is contracted at the high speed.
However, in the piezoelectric actuator of the above references, the piezoelectric element is disposed at only one end of the drive shaft. When a distance between the lens holding member and the piezoelectric element is increased, the displacements of the piezoelectric element cannot be transmitted efficiently. As a result, a moving distance of the lens holding member is reduced. Further, since only one drive shaft is provided, plural lens holding members, which are to be moved in different directions, cannot be disposed.